A confirmation text is the message that makes a booking real. The client asked for a time, by phone, at the desk or on a booking page; the business checked the diary and replies with what it can offer. A good one carries the day, the time, the service, the price if it is not obvious, and any rule that applies, such as a deposit or what to bring. It is short enough to read at a glance and complete enough to settle a later disagreement.
The text is also your record. If a client says she was booked for 11 and not 10, the message both of you can scroll back to answers it without an argument.
In practice
A lash studio confirms every request with the same pattern: "Hi Ana, you're booked for a classic fill on Thursday at 6 pm, $75, about an hour. Reply here if you need to change it." It takes under a minute to send between clients. Over a month the studio confirms 64 requests this way and has 2 disagreements about times, both settled by the message history.
Send it fast. A request confirmed within a couple of hours feels like one conversation to the client; one confirmed the next day feels like a second attempt to book. If you take deposits, the confirmation is the natural place to ask, because the client has just been given what they wanted.
A confirmation is different from a reminder text, which comes a day or two before the appointment itself.
In Glow Line
Glow Line never sends the confirmation for you. Each request on the Bookings page shows the client's mobile as a link that opens a text, so you write and send the confirmation yourself once you have checked the diary. See confirm a booking request.